The invention is directed to the fabrication of steel wire of various types by a method which is capable of producing a quality product at high rates of productivity. The method, which is a complete departure from conventional practice, has particular significance when used in the manufacture of filamentary steel wire of very fine diameter, for example, in the range of from about 0.5 to 20 mils.
Although the conventional practice for manufacturing steel wire is capable of producing a high quality product, the need for casting, repeated mechanical reductions, intervening heat treatments and other required operations render the resulting wire product relatively expensive. This becomes apparent when one considers the steps involved merely to obtain the intermediate steel wire rod product. That is, molten steel is cast into ingots which are subsequently rolled into blooms from which billets are formed. Finally, the billet is hot rolled to produce the steel wire rod. The wire rod must then undergo a series of elaborate and costly metal-drawing and heat treating operations to obtain a steel wire product of the desired cross-section and mechanical properties.
It is, of course, obvious that the smaller the diameter of the wire produced by the afore-mentioned processing procedures, the greater will be cost of production. Yet, there has been an increasing demand for wire having diameters in the range of 10 mils or less, and in some applications less than 5 mils is desired. This demand has come about largely as a result of the growing use of filamentary steel wire as a reinforcing element in composite materials. For example, fine diameter steel is now widely used to reinforce the rubber carcass of pneumatic tires. The steel tire cord employed for this purpose is generally made from high carbon steel, i.e., from 0.6 to 0.8 percent by weight of carbon. Because of the relatively low product yield, wire drawing of such high carbon material to attain fine diameters becomes excessively expensive. Moreover, because of the loss of ductility resulting from the need to pull the wire through numerous drawing dies, frequent intermediate heat treating steps are needed to restore the ductility required for further drawing.
There is, therefore, a desire and a need for an alternative to the conventional practice for producing steel wire wherein a product of substantially equivalent properties can be produced at considerably less cost.
A number of previous attempts have been made to meet this need, but for one reason or another the methods proposed have not proved to be entirely successful in practice. Perhaps, most noteworthy of the prior proposals is a method wherein certain techniques of the ceramic arts are utilized. Such method is described in U.S. Pat. No. 3,671,228 and involves a procedure wherein a powdered agglomerate of iron oxide is mixed with a binder and the mixture is placed in a die chamber where it is compacted and extruded with a hydraulic press to form filaments. The filaments are then subjected to a reducing atmosphere at a temperature below the sintering temperature to effect reduction of the metal oxide to the metallic state followed by a sintering of the reduced compact to form wire.
Although this prior method constitutes a significant advance in the art, the brittleness of the precursor filaments makes further handling in the conversion operations difficult. Moreover, the high pressures required to form the precursor add to processing costs.
It is, therefore, an object of this invention to provide an entirely new approach to the production of filamentary wire.
It is further object of this invention to provide a method for producing filamentary steel wire which is substantially less costly than the conventional practice.
It is a still further object of this invention to provide a method for producing steel wire products which have outstanding mechanical properties.